This year's brood of 17-year cicadas will affect states stretching from North Carolina to New York. / Gerry Broome, AP

by John Waggoner, USA TODAY

by John Waggoner, USA TODAY

The first of the 17-year cicadas have emerged from their underground lairs and, after shedding their skins and some brief flying lessons, they're checking their portfolios. There are about 1 million cicadas per 2.5 acres, and their buzzing can be translated, roughly, as "wait â?¦ what?"

Back when they were born, in 1996, their parents told them to invest in stocks, which had earned an average annual return of 10.7% since 1926, according to Ibbotson & Associates. And, in fact, that seemed cautious: The parents of the current Brood II cicadas earned 15.9% a year on their investments in the Standard & Poor's 500-stock index, and 13.7% annually on the Nasdaq 100 index.

Brood II's parents were just magicicada magicians when it came to money. They earned 10.9% a year on their 10-year Treasury notes, and 7.7% from their ultrasafe three-month T-bills. Naturally, they wisely reinvested dividends and interest.

The current crop, however, weren't nearly as lucky. Their 17-year average annual returns:

Fortunately for Brood II, their retirement is just a few weeks. And they experienced a bit less inflation over their 17 years than their parents did. Inflation has averaged 2.4% a year for the past 17 years, vs. 4.8% for 1979-1996.

So, what can we learn from the investing fortunes of our periodic visitors? The first is that many investors base their expected returns on the long-term average, or mean, returns from an asset class - the kind that Ibbotson produces every year. According to theory, above-average returns eventually revert to the mean.

The problem, as our insect friends have discovered, is that it can take a very long time for averages to revert to the mean. As it happens, the current brood of cicadas has been investing for about as long as parents of near-college-age students have been investing - and, by and large, both parents and children are probably feeling a bit cheated, especially given the long-term averages and soaring college costs.

The other problem, of course, is that there's no law saying that returns must return to the mean. If there were Japanese 17-year cicadas, for example, they would have a much different view on the cyclical nature of investing.

The mean also varies according to where you begin and end. As you might suspect, the mean return from stocks has been slowly slipping every year the current brood of cicadas has been underground. The mean return from the S&P 500 has fallen to 10% from 1926 to 2013, vs. 10.7% from 1926 to 1996, according to Ibbotson.

Even more important for investors is the powerful effect on psychology that would affect how one brood made investment choices, says Barry Glassman, a McLean, Va., financial planner. For example, the parents of the current brood would have made darn good money in the stock market. If they had listened to their parents - the grandparents of the current brood - they would have treated stocks like the Orkin man.

The brood that emerged in 1979 would have witnessed one of the worst markets in history, Glassman says. The S&P 500 with reinvested dividends gained just 6.5% a year during Grandma and Grandpa Cicada's life span. Inflation averaged 5.1%. Grandma and Grandpa would probably have pointed their children toward Treasury bonds, which yielded 9.3% in May 1979, or even Treasury bills, which averaged 10% that year.

So perhaps the biggest lesson long-term investors can learn from our 17-year visitors is this: You really don't know what will happen so far in the future, no matter what the long-term averages tell you. Your best defense, Glassman says, is a diversified portfolio of stocks, bonds, money funds and commodities. If you can lift your head out of the ground every so often and rebalance your portfolio, all the better.